Blueberry Lemon Poppy Seed Custard Cake

Preheat oven to 325°F (160°C). Grease and line an 8×8-inch baking pan with parchment paper.
In a medium bowl, whisk together flour, baking powder, salt, and poppy seeds. Set aside.
In a large bowl, beat egg yolks with sugar until light and creamy. Mix in melted butter, lemon zest, and lemon juice.
Gradually add the flour mixture to the wet ingredients, then slowly whisk in the lukewarm milk until smooth. The batter will be thin.
In a separate bowl, beat egg whites until stiff peaks form. Gently fold egg whites into the batter in three additions. Some lumps are normal.
Pour batter into the prepared pan. Scatter blueberries evenly on top.
Bake for 45–55 minutes, until the top is golden and the center jiggles slightly.
Cool completely, then refrigerate for at least 2 hours to set the custard layer.
Dust with powdered sugar before serving if desired.